1. Field of the Invention
The present invention relates to a novel photocatalyst useful for a photoreaction in which hydrogen is produced from water and to a method for the preparation of the same.
Also, the present invention is concerned with production of hydrogen with the photocatalyst in combination with a promoter.
2. Description of the Prior Art
Hydrogen is an industrially very useful material over a wide range. For example, synthesis of ammonia and production of hydrogen chloride require hydrogen. It is essential for hydrogenation reactions for the conversion of unsaturated compounds into saturated compounds. In addition, hydrogen is used in the hydrotreating processes, such as hydrogen addition, desulfurization, denitrogenation and demetallization, with the aim of improving the quality of petroleum products. Hydrogen is also ecologically useful. For example, carbon dioxide, a compound which causes the warming of the globe, can be reused by application of the contact hydrogenation of carbon dioxide for which hydrogen is indispensable. Further, hydrogen occupies a powerful position of a pollution-free energy source, substituting the existing fossil fuels.
Many methods have conventionally been undertaken in order to obtain hydrogen, for example, acquirement from fossil fuels, modification of natural gases, reaction of iron with steam, reaction of water with metals, electrolysis of water and etc. Such conventional methods, however, are uneconomical because an enormous quantity of thermal or electric energy is consumed. In addition, oxygen and carbon oxide, the by- products, are produced in large quantities, giving rise to a danger of explosion as well as pollution of environment. Further, the conventional methods are generally poor in production yield and purity of the product. The complexity of process in above-mentioned methods gives another disadvantage. The production of hydrogen by the conventional techniques is carried out at relatively high temperatures, which requires thermal-resistant and thus very expensive gas furnaces and facilities. Accordingly, such high expenditure of facility investment is an obstacle in industrializing the techniques.
Most hydrogen is in a form of water or inorganic substance. Hydrogen gas easily frees itself from the gravity field due to its low mass and thus is present in a very small amount in the air. It is therefore very important to develop a technology for preparing hydrogen from water in high purity and efficiency, in a view of solving the urgent problem of substitute energy source and securing the material of chemical industry.
As efficient techniques for preparing hydrogen from water, there are photoreactions of water using photocatalysts, which have recently been developed. Not much of prior techniques concerning photocatalyst for preparing hydrogen are known.
Japanese Patent Publication Laid-Open No. Sho. 62-191045 discloses that hydrogen is generated by the photolysis of a Na.sub.2 S aqueous solution in the presence of a photocatalyst of rare-earth element compound. This photocatalyst is advantageous in its catalytic activity with visible ray.
Japanese Patent Publication Laid-Open No. Sho. 63-107815 uses a composite oxide of niobium and alkali metal as a photocatalyst. Hydrogen is produced by a photolytic reaction in methanol aqueous solution in the presence of the photocatalyst. There is also an advantage that the catalyst is active to visible ray. However, the above-cited patents are disadvantageous in that the amounts of hydrogen generated are very small.
Japanese Patent Laid-Open Publication No. Hei. 1-208301 reveals an advantage of high efficiency for hydrogen generation by thermal-reacting water with aluminum. However, a large quantity of thermal energy is consumed as the thermal reaction occurs at high temperatures of more than 600.degree. C.